You are given a geometric progression {bn}. Calculate the sum of the first 3 terms if b3 = 8, q = -2.

Since a geometric progression is a sequence in which each subsequent number, starting from the second, is obtained from the previous one by multiplying it by the denominator q.
Formula of the n-th member: bn = b1 * q to the power (n-1).
Using b3 and q:
b3 = b1 * q degree 3-1
b3 = b1 * q to the power of 2;
8 = b1 * (- 2) to the power of 2;
8 = b1 * 4
b1 = 8: 4
b1 = 2
Sum formula: Sn = b1 (q to the power of n –1) / q – 1.
Sn = b1 (q to the n-1 power) / q -1;
S3 = b1 (q to the power of 3 –1) / q – 1;
S3 = 2 ((- 2) to the power of 3 – 1) / – 2 – 1 = 2 * (- 8 – 1) / – 3 = 6